1. Field of the Invention
The present invention relates to an apparatus for detecting the coordinate position of a contact point on a resistance layer type touch panel.
2. Related Arts
The touch panels of this type are composed of two resistance layers opposed to each other and separated by a certain gap. One resistance layer has a pair of electrodes disposed on its edges to be opposed in a X direction and the other resistance layer has a pair of electrodes disposed on its edges to be opposed in a Y direction. These resistance layers are alternately charged with electric current through their electrodes to measure a voltage induced to the resistance layer that is not being charged with the electric current. The coordinate position (x and y) of a contact point on the touch panels is detected by measuring the induced voltage.
Such alternate supply of electric current to the resistance layers during input stand-by periods wastes electric power. Therefore, the touch panels of this type are so designed that the electric current supply to these resistance layers is suspended during the input stand-by periods and an occurrence of a depressing operation is automatically detected.
An example of such touch panels is described in Japanese Laid-open Patent Application No. 4-352219. According to the touch panel of the example, as shown in FIG. 1A, a pair of rectangular resistance layers 21 and 22 are disposed to be opposite each other with a certain gap therebetween. The resistance layer 22 is provided with a pair of electrodes a2 and a3 disposed at its edges to be opposed in a X direction. The other resistance layer 21 is provided with a pair of electrodes a1 and a4 disposed at its edges to be opposed in a Y direction which is perpendicular to the X direction. Each of the electrodes a1 and a3 is connected with a respective power source Vcc having a fixed voltage via a transistor Tr1 and Tr3 respectively. Each of the electrodes a2 and a4 is earthed via a transistor Tr2 and Tr4 respectively, and the voltage fluctuations of these electrodes a4 and a2 are indicated respectively as a position detection signal V.sub.x in X direction and a position detection signal V.sub.y in Y direction. In the input stand-by periods, all the transistors Tr1-Tr4 are put in an off-state and the anode of a diode D is set in a high level, thereby realizing an equivalent circuit shown in FIG. 1B.
In the equivalent circuit, as soon as the two resistance layers 21 and 22 contact with each other at a contact point as a result of being depressed, electric current is supplied to the resistance layers 21 and 22 and a resistance R down to the ground. As a result, the voltage level of the electrode a2 is changed, so that a control unit (not shown) can detect an occurrence of a depressing operation. After the detection, the control unit controls the alternate supply of electric current between the transistors Tr1 and Tr4 for the resistance layer 21 and the transistors Tr2 and Tr3 for the resistance layer 22.
However, these conventional touch panels have a problem as follows. The level of a voltage to be induced to the electrode a2 greatly differs depending on a depressed point during the input stand-by periods in addition to the contact resistance between the resistance layers 21 and 22. These conditions make it extremely difficult to detect an occurrence of a depressing operation.
For example, when a point in the neighborhood of the electrode a1 of the resistance layer 21 and in the neighborhood of the electrode a2 of the resistance layer 22 is depressed and as a result, these resistance layers 21 and 22 contact with each other, a value of a voltage (Va2) induced to the electrode a2 is given by: ##EQU1## where r is a contact resistance between the resistance layers 21 and 22.
On the other hand, when a point in the neighborhood of the electrode a3 of the resistance layer 22 and in the neighborhood of the electrode a4 of the resistance layer 21 is depressed, the value of voltage (Va2) is given by: ##EQU2## where R.sub.21 is a resistance value between the electrodes a1 and a4, and R.sub.22 is a resistance value between the electrodes a2 and a3.
Generally, these resistance values R.sub.21 and R.sub.22 are several hundred ohms (for example 200.OMEGA.), and the contact resistance r changes in a range between 0 and 10 k.OMEGA. in accordance with the strength of a depression.
In these equations (1) and (2), even if the contact resistance r changes in the above-mentioned range, as far as the fixed resistance R is much larger than the sum of the maximum value of the contact resistance r and the two resistance values R.sub.21 and R.sub.22, the detection of an occurrence of a depressing operation becomes possible.
For example, if EQU R&gt;&gt;r+R.sub.21 +R.sub.22 ( 3)
then, even when a depressed point is on a very edge of the resistance layer as described in the equations (1) and (2), an occurrence of the depression can be detected from the change of Va2 as follows. EQU Va2.apprxeq.Vcc (4)
However, to set a large value as R is accompanied by an inconvenience as follows.
The detection of an occurrence of a depressing operation is executed even when an operator slightly touches the resistance layer without intention or when a chattering is caused before or after an actual depressing operation by a transitional vibration of either resistance layer. The chattering means, as shown in FIG. 2, that the resistance layers 21 and 22 are alternately put in a state of being in touch and the other state of being out of touch in a short cycle.
In order to solve the inconvenience, a value of the fixed resistance R must be small to some extent; however, this condition causes the voltage Va2 to greatly vary depending on a depressed point. Hence, it becomes impossible to distinguish a depressing operation in the neighborhood of the electrodes a3 and a4 from a slight touch with no intention of a depression.